As an example, I'm looking for a data scientist job and as you might guess, it is a very broad field. So, I thought I would learn the basics of machine learning, deep learning, required maths and programming and then I would search for the job and in whatever field I'll get the job like Natural language processing, computer vision, supply chain analytics, time series analysis, etc. , I would learn that and specialize in that field. But when I started looking for job every start-up wanted that I know current state-of-the-art techniques of their field, which is simply not possible as each start-up specialize in a different set of techniques, for example, some want deep learning, some statistics, some time series analysis, some big data processing frameworks like Apache Spark, some recommendation system and so on and so forth. As a result, I'm still searching for a job and somewhat have lost passion for data science.
So, the only learning that I got from going through this process is whatever field you choose, you have to specialize in that field to have a great career.